Searching for absorbed AGN in the 2XMM-Newton pre-release EPIC Serendipitous Source Catalogue

0.34 MB·English

by  Elisabetta Memola

Astronomy&Astrophysicsmanuscriptno.memola˙2xmm˙paper (cid:13)c ESO2008
February5,2008
Searching for absorbed AGN in the 2XMM-Newtonpre-release
∗
EPIC Serendipitous Source Catalogue
7
0 E.Memola,1 A.Caccianiga,1 F.Cocchia,1 R.DellaCeca,1 T.Maccacaro,1 P.Severgnini,1 D.J.Fyfe,2 S.Mateos,2
0
M.G.Watson,2 andG.Lamer3
2

n
a 1 INAF-OsservatorioAstronomicodiBrera,ViaBrera28,20121Milano,Italy
J 2 DepartmentofPhysics&Astronomy,UniversityofLeicester,LeicesterLE17RH,UK

4 3 AIP-AstrophysikalischesInstitutPotsdam,AnderSternwarte16,14482Potsdam,Germany
2

1
ReceivedOctober15,2006;acceptedDecember29,2006
v
5
ABSTRACT
7
6
1 Aims.WeaimtotestamethodofefficientlyselectingX-rayobscuredAGNinthe2XMM-NewtonEPICSerendipitousSourceCatalogue.
0 Methods.BymeansofastrongcorrelationestablishedusingtheXMM-NewtonHardBrightSamplebetweentheintrinsicabsorptionandthe
7 hardnessratiotothe0.5–2.0keVand2.0–4.5keVbands,anefficientwayofselectingabsorbedsourceshasbeenworkedout.Ahardnessratio
0 selectionbasedonthe2XMM-Newtonpre-releaseSourceCatalogueledustothedefinitionofcandidateslikelytobeobscuredinX-rays.
/ Results.X-ray and optical spectral analysis were performed for three objects. Strong absorption (N > 1022 cm−2) was detected from the
h H
X-rayanalysis, confirming the efficiency of the method used to select obscured sources. The presence of absorption isalso revealed in the
p
- opticalband,althoughatasignificantlylowerlevelthaninferredfromtheX-rayband.
o
r
t
s Keywords.Surveys-Methods:dataanalysis-X-rays:galaxies-Galaxies:active
a
:
v
i
X 1. Introduction cases even to reveal (e.g. DellaCecaetal.2002), their nu-
r clear properties in the optical and soft X-ray (E<2 keV)
Medium-deep X-ray surveys carried out with Chandra and
a bands. Absorbed AGN can be efficiently selected (at least
XMM-Newton (see Brandt&Hasinger2005 and references
the Compton-thin ones) and studied using hard X-ray (E∼2–
therein for a recent review) have shown that obscured active
10keV)bandswheretheeffectofabsorptionislesssevereand
galacticnuclei(AGN)arethemaincontributorstothecosmic
where the host galaxy contribution is minimal. Nevertheless,
X-raybackground(XRB).Fabian&Iwasawa(1999)foundout
as shown e.g. by the XMM-Newton hard brightsample, HBS
thatatleast85%oftheemittedintrinsicfluxoftheXRBspec-
(Caccianigaetal.2004; DellaCecaetal.2005; Della Ceca et
trum is absorbed, by assuming the absorber to be dusty gas.
al.2007,inpreparation)atfluxesabove∼ 7×10−14 ergcm−2
Therefore,understandingthecharacteristicsofobscuredAGN
s−1,eveninthe“hard”4.5–7.5keVband,absorbedAGN1rep-
(i.e.luminosityandredshiftdistribution,spectralenergydistri-
resent onlythe ∼ 25%of the sources. It is thereforeessential
bution,thepropertiesofthecircumnuclearmedium,cosmolog-
to increasetheefficiencyin selectingX-rayabsorbedAGNin
icalevolution)isakeyissueinunveilingtheaccretionprocess
ordertobuildupwell-definedandsizablesamplesofthesein-
onsupermassiveblackholesandtracingandfollowingitsevo-
terestingsources.
lutionwithcosmictime.
Here we introducea powerfulmethodfor selecting bright
To efficiently select sizeable and representative samples
X-rayabsorbedcandidatesanddiscusstheresultsoftheX-ray
of absorbed AGN is not an easy task, even at bright fluxes.
andopticalspectralanalysisforthreesources.Throughoutthis
The strong absorption that affects the intrinsic emission of
paperweadoptH =70kmsec−1Mpc−1,Ω =0.3,andΩ =
these sources does not allow us to study, and in several o M Λ
0.7.Wedefineapower-lawspectrumsuchthatdN/dE=AE−Γ
Sendoffprintrequeststo:E.Memola whereNisthenumberofphotons,Ethephotonenergy,Athe
email:[email protected] normalization,andΓthephotonindex.
∗ Based on observations performed at the European Southern
Observatory,LaSilla,ChileandonobservationsobtainedwithXMM-
Newton,anESAsciencemissionwithinstrumentsandcontributions 1 WeusehereandinthefollowingN =1022 cm−2asthedividing
H
directlyfundedbyESAMemberStatesandtheUSA(NASA). valuebetweenunabsorbedandabsorbedsourcesintheX-rays.
2 E.Memolaetal.:SearchingforabsorbedAGNinthe2XMM-Newtonpre-releaseEPICSerendipitousSourceCatalogue∗
2. Bright,obscuredAGNcandidatesinthe moregeneralprogrammeconcerningtheHardBrightSample.
2XMM-Newtonpre-releaseEPICSerendipitous Inthefollowingwepresentanddiscussourresults.
SourceCatalogue
The work presented here is based on the results from the 3. X-rayspectralanalysis
XMM-Newton Hard Bright Sample (Caccianigaetal.2004;
DellaCecaetal.2004; DellaCecaetal.2005). The survey We performed the X-ray spectral analysis of XMM-Newton
was performed in the 4.5–7.5 keV band at a flux limit of data for the following sources: 2XMMpJ115121.8−283605,
F ∼ 7 × 10−14 erg cm−2 s−1. The source sample is now al- 2XMMpJ145442.2+182937,and2XMMpJ151612.2+070341.
x
most fully spectroscopically-identified (65/67 sources iden- TheywereobservedbetweenAugust2000andFebruary2003
tified), and the optical and X-ray spectral analysis is com- withexposuretimesrangingfrom∼23upto∼42ksec(forthe
plete(DellaCecaetal.2005;Caccianigaetal.2007,inprepa- observationdetailsseeTable1).TheEPICMOS1,MOS2,and
ration; Della Ceca et al. 2007, in preparation). The authors pndetectorswereoperatedinPrimeFullWindowMode.
found that X-ray absorbed and unabsorbed AGN are clearly All MOS and pn data were reprocessed with the XMM-
distinguishable by means of the hardness ratio HR2, defined Newton Science Analysis Software (XMM-SAS) ver-
as HR2 = CR3−CR2, where CR2 and CR3 are the PSF and sion 6.5.0, using the latest calibration products. Source
CR3+CR2
vignetting-correctedcountratesinthe0.5–2.0and2.0–4.5keV counts for MOS1, MOS2, and pn (when available)
energyband,respectively(seeFig.3inDellaCecaetal.2005). were extracted in the 0.2–12 keV band from a circu-
By considering the constraint HR2 > 0, at the survey flux lar region, centred on the source, which had a radius
limit,theyfound14sourcesoutof15(∼93%)tohaveabsorb- of 25 arcsec for 2XMMpJ115121.8−283605, 14 arc-
ing column densities higher than ∼1022 cm−2, and 4 of these sec for 2XMMpJ145442.2+182937, and 12 arcsec for
sources(∼30%)turnedouttobetherareX-raytype2quasars 2XMMpJ151612.2+070341, depending on the countrate
(L >1044ergs−1;N >1022cm−2). statistics and source off-axis. Background spectra were ex-
x H
The XMM-Newton Survey Science Centre (SSC)2 is tracted from larger source-free circular regions (radius of ∼
assembling the second XMM-Newton EPIC Serendipitous 40−80 arcsec) close to the object. We selected single and
Source Catalogue (Watson et al. 2007, in preparation). A double events (PATTERN≤4) for the pn and single, double,
first preliminary version of the 2XMM Serendipitous EPIC triple, and quadruple events (PATTERN≤12) for the MOS.
SourceCatalogue(2XMMp)hasbeenrecentlyreleasedandis OuranalysisisbasedonbothEPICMOS(MOS1andMOS2)
available at http://xmm.vilspa.esa.es/xsa/. The catalogue con- and pn data. In particular, for both MOS1 and MOS2, we
tains 153105 X-ray source detections drawn from 2400 use- extracted source and background spectra and then summed
fulXMM-NewtonEPICpublicobservations.Themedianflux themupaposterioribymeansofmathpha,endingupwithone
(inthetotalphotonenergyband0.2−12keV)ofthecatalogue sourcespectrumandonebackgroundspectrumregionvalidfor
sources is ∼ 2.4× 10−14 erg cm−2 s−1; ∼20% of the sources a global MOS1+MOS2 case. The ancillary response matrix
havingfluxesbelow10−14ergcm−2s−1. (ARF) and the detector response matrix (RMF) were created
bytheXMM-SAStasksarfgenandrmfgen.Thearfgenroutine
The values of HR given in the catalogue are defined us-
convolvesthemirroreffectivearea(vignettingmodified),filter
ingenergybandsthatareslightlydifferentfromtheonesused
transmission, and CCD efficiencies as a function of energy
in the HBS. The HR closer to the HR2 used in the HBS (see
above) is the HR3, defined as HR3 = CR4−CR3, where CR3 to return the ARF component of response. The ARF, at the
CR4+CR3 sourcepositiononthedetector,isemission-weighted,andthe
and CR4 are the PSF and vignetting-corrected countrates in
weighting falls off with distance from the source in line with
the1.0−2.0and2.0−4.5keVenergyband,respectively.Inthe
2XMMp catalogue, there are 427 high latitude (|b| > 20◦) the PSF. The RMF is spatially averaged using an appropriate
sources,whichhaveHR33>0andMOS2detectionlikelihood PSFdetectormap.Theresponsematrixeswerealsomergedby
using addrmfandaddarf.Due to the countrateof oursources
(ML) in the 4.5–12 keV energyband > 20 (Cash1979). This
(seeTable1),pileupisnotaproblem.
groupof sourcesrepresentsa crucialstarting pointfortesting
theefficiencyinselectingabsorbedAGN.We pickedupthree We used the XSPEC package (version 11.3.1;
brightX-ray sourceswith F > 10−13 erg cm−2 s−1 in the en- Arnaud1996) in order to perform the spectral fit-
x
ergy band 4.5–12 keV and a bright optical counterpart (R < ting analysis. MOS and pn spectra were fitted simul-
17 mag) to analyse their X-ray and optical spectra. The op- taneously (when both data were available) and re-
tical counterparts were observed at the 4-m New Technology spectively in the 0.3–10 (2XMMpJ115121.8−283605),
Telescope(NTT@ESO)inChilelastMarch2006,aspartofa 0.7–10 (2XMMpJ145442.2+182937), and 0.5–10
(2XMMpJ151612.2+070341) keV energy range. The source
counts were grouped into energy bins such that each bin
2 The XMM-Newton Survey Science Centre (SSC, see
contains at least 15 counts where the fit statistic in use was
http://xmmssc-www.star.le.ac.uk) is an international collabora-
chi-squared(χ2). The quotederrorson the best-fit parameters
tion,involvingaconsortiumof10institutions,appointed byESAto
correspond to the 90% confidence level for one interesting
helptheSOCindevelopingthesoftwareanalysissystem,topipeline
process all the XMM-Newton data, and to exploit XMM-Newton parameter(i.e.,∆χ2 =2.71;Avni1976).ToanalysetheX-ray
serendipitousdetections. data we made use of the redshifts derived from the optical
3 HR3refersheretoMOS2data,whichwerealsousedfortheHBS. spectra(see§4andTable2).
E.Memolaetal.:SearchingforabsorbedAGNinthe2XMM-Newtonpre-releaseEPICSerendipitousSourceCatalogue∗ 3
3.1.2XMMpJ115121.8−283605
The source 2XMMpJ115121.8−283605 (see Fig. 1) was ob-
servedbyXMM-NewtononJanuary3,2002forabout40ksec
of good exposure time. The pn data are not available be-
cause the source is outside the pn field of view. Fitting the
data with a single absorbed power-law model leads to ob-
vious residuals in the low-energy range. Our best-fit model
(χ2/dof = 27.41/25) is therefore based on a leaky-absorbed
power-lawcontinuum(seeTable2),wheretwopower-lawsare
invoked to explain both the soft (0.3−1.0 keV - unabsorbed
power-law) and the hard (up to 10 keV - absorbed power-
law) part of the X-ray spectrum. The leaky-absorbed power-
law continuum has a functional form, which provides a sim-
ple and convenient parametrization of the relative contribu-
tionsofboththeprimaryabsorbedcomponentandthecompo-
nentthatproducesthefluxexcessatsoftenergies.Themodel Fig.1. 2XMMpJ115121.8-283505: XMM-Newton MOS
mightrepresentthephysicalcaseofanX-raysourceobserved folded spectrum. The best-fit model is a leaky-absorbed
both directly through the absorbing torus (absorbed power- power-law.
law)andafterthescatteringonawarm,highlyionizedgaslo-
cated outsidethe absorbingmedium(unabsorbedpower-law).
thedata,thereforewealsocannotexcludeapossibleblending
Another possible interpretation is the presence of a cloudy- of two narrow iron lines at 6.4 and 6.7 keV. The X-ray lu-
absorbing medium, which would be responsible for both the minosity(L ∼0.7×1043ergs−1)istypicalofaSeyfertgalaxy.
x
absorbed and the transmitted component.Nevertheless, emis-
sionlinesatsoftX-raysmightbe present,duetonuclearpro-
cesses or related to starburst activity, as well as a thermal
3.3.2XMMpJ151612.2+070341
component because of the presence of a hot plasma. We can
clearly not prove this hypothesis due to the lack of soft en- The source 2XMMpJ151612.2+070341 (see Fig. 3) was ob-
ergy statistics. The two power-laws are constrained to have
servedbyXMM-NewtononAugust21,2000forabout30ksec
the same photon index (Γ ∼ 1.9), whereas the normalization
ofgoodexposuretimewiththeMOScamerasand23ksecwith
of the unabsorbed componentresults in a few percent (∼3%) thepncamera.Asingle-absorbedpower-lawmodel(χ2/dof =
of the normalization of the absorbed one, in agreement with
18.98/12) provided a best-fit value of the photon index that
previous results on absorbed AGN (e.g. Turneretal.1997;
wasverylow(Γ ∼ 0.5),whereasthevalueofN wasalready
H
DellaCecaetal.2000; Caccianigaetal.2004). The absorbed highandabout∼4×1022cm−2.SuchaflatΓvaluewouldsug-
power-law reveals an intrinsic hydrogen column density, N ,
H gest Compton thickness, a thesis otherwise not supported by
of about 2.6×1022cm−2, which clearly points to an X-ray ab- theopticaldata.AfterfreezingthevalueofΓto1.9,moretypi-
sorbed source (even with a single-absorbedpower-law model
calofAGN,therewereresidualsat1–2keV.Thebest-fitmodel
theNHwasabout2×1022cm−2).TheintrinsicX-rayluminosity (χ2/dof =16.55/12)isaleaky-absorbedpower-lawcontinuum
ofthissource(L ∼1.8×1043ergsec−1)istypicalofaSeyfert
x with a frozenphotonindexof1.9(see Table2).Thestatistics
galaxy.
donotallowustoleavethisparameterfree.Thehydrogencol-
umndensityisN ∼1.4×1023cm−2,whichpointstoanX-ray
H
absorbed object. The X-ray luminosity (L ∼ 1.6×1043 erg
3.2.2XMMpJ145442.2+182937 x
s−1)istypicalofaSeyfertgalaxy.
The source 2XMMpJ145442.2+182937 (see Fig. 2) was
observed by XMM-Newton on February 16, 2003 for about
4. Opticalspectralanalysis
37 ksec of good exposure time by the MOS cameras and 25
ksecbythepncamera.Thebest-fitmodel(χ2/dof =30.73/33) We performedthespectralanalysisoftheopticalcounterparts
basedonbothMOSandpndataisasingleabsorbedpower-law for the threeX–raysources. Opticalimagesandspectroscopy
along with a Gaussian line detection at 90% confidence level were acquired at the NTT@ESO telescope using EMMI. We
(see Table 2). The power-law shows a photon index Γ ∼ 1.4, obtained shallow optical images with a typical exposure time
togetherwithN ∼ 1.5×1022 cm−2,whichpointstoanX-ray of 60–90s. Optical images were bias-subtracted, flat-field di-
H
absorbedsource.IfwefreezethephotonindextoΓ ∼ 1.9,the vided,andflux-calibratedusingobservationsofstandardstars
N valueremainsquitestablearound∼2.4±0.4×1022cm−2.In obtained during each night. Astrometric calibration was per-
H
bothcasestheironlineequivalentwidthisabout700eVwith formed using the software package GAIA (version 2.6-9 by
associate errors of about 60%. This equivalent width value, P.W. Draper) by matching sources found in the USNO cat-
due to the large error associated to it, is consistent with the alogue (Monet et al. 1998). We show the R-images of our
value of N measured from the cut-off. Moreover, a detailed sources in Figs. 4, 5, and 6 (left panels) and report their R-
H
analysisofthelinepropertiesisnotwarrantedbythequalityof magnitudesinTable2.
4 E.Memolaetal.:SearchingforabsorbedAGNinthe2XMM-Newtonpre-releaseEPICSerendipitousSourceCatalogue∗
Table1.XMM-Newtonobservationlog.
Source Obsid Obs.Date mos1-exp(s) mos2-exp(s) mos1+mos2-cts pn-exp(s) pn-cts Filter∗
2XMMpJ115121.8−283605 0027340101 20020103 42062 37253 299 nopn nopn Thin1
2XMMpJ145442.2+182937 0145020101 20030216 36975 36421 280 25535 274 Thin1
2XMMpJ151612.2+070341 0109920101 20000821 30076 30123 84 23085 133 Thin1
(∗)FiltertypereferstoalltheEPICinstrumentsinvolvedinthecorrespondingobservation.
Table2.XMM-Newtonspectralanalysis:best-fitparameters.
Source R z N(a) N(b) Γ E EW χ2/dof Flux-mos(c) L(d)
HGal H Fe−Kα Fe−Kα
mag keV eV
2XMMpJ115121.8−283605(1) 16.64 0.177 0.06 2.6+1.4 1.9±0.5 – – 27.41/25 1.76 1.8
−1.2
2XMMpJ145442.2+182937(2) 16.12 0.116 0.02 1.5+0.8 1.4±0.3 6.34+0.44 700+300 30.73/33 2.08 0.7
−0.5 −0.11 −480
2XMMpJ151612.2+070341(1) 17.20 0.176 0.03 14+7.4 1.9∗ – – 16.55/12 1.07 1.6
−4.4
(1)Model:Leaky-absorbedpower-law;(2)Model:Single-absorbedpower-law+Gaussianline;(∗)Frozen;(a)Galacticcolumndensityinunitsof
1022cm−2;(b)Absorbingcolumndensityinthesourcedirectioninunitsof1022cm−2;(c)X-rayfluxesintheband2-10keVinunitsof10−13ergs
cm−2sec−1andcorrectedonlyfortheGalacticabsorption;(d)X-rayluminositiesintheband2-10keVinunitsof1043ergssec−1andcorrected
forGalacticandintrinsicabsorption.
Fig.4. 2XMMpJ115121.8−283605:NTT@ESO R-image (1’×1’). North is on top and east to the left. A circle of 5” radius,
correspondingtothe95%confidencelevelX-rayerrorcircle(seeDellaCecaetal.2004),isshown(leftpanel)toclearlymark
theopticalcounterpartoftheXMM-Newtonsource;NTT@ESOopticalspectrum(rightpanel).
E.Memolaetal.:SearchingforabsorbedAGNinthe2XMM-Newtonpre-releaseEPICSerendipitousSourceCatalogue∗ 5
Fig.5. 2XMMpJ145442.2+182937:NTT@ESO R-image (1’×1’). North is on top and east to the left. A circle of 5” radius,
correspondingtothe95%confidencelevelX-rayerrorcircle(seeDellaCecaetal.2004),isshown(leftpanel)toclearlymark
theopticalcounterpartoftheXMM-Newtonsource;NTT@ESOopticalspectrum(rightpanel).
Fig.6. 2XMMpJ151612.2+070341:NTT@ESO R-image (1’×1’). North is on top and east to the left. A circle of 5” radius,
correspondingtothe95%confidencelevelX-rayerrorcircle(seeDellaCecaetal.2004),isshown(leftpanel)toclearlymark
theopticalcounterpartoftheXMM-Newtonsource;NTT@ESOopticalspectrum(rightpanel).
6 E.Memolaetal.:SearchingforabsorbedAGNinthe2XMM-Newtonpre-releaseEPICSerendipitousSourceCatalogue∗
strong emission lines, like the [OIII]λ5007Å and the Hα-
[NII] blend, and a continuum that is highly contaminated (or
dominated) by the light of the host galaxy. The presence of
broad (FWHM∼1900−3000 km/s) emission lines is clear in
onesource(2XMMpJ151612.2+070341)andisalsosuggested
in2XMMpJ115121.8−283605,wheretheHαislikelytohave
a broad component. In 2XMMpJ145442.2+182937, instead,
onlynarrowlinesarevisibleinthespectrum.
To estimate the level of optical absorption in the three
sources,wemadeuseofthemethoddescribedinCaccianigaet
al.(2007,inpreparation).Theyapplythisproceduretostudy-
ing and classifying the objects in the XMM-Newton Bright
SerendipitousSurveythatarecontaminatedbythehostgalaxy
light (“diluted” sources). In synthesis, this approach involves
a spectral model composed of a galaxy plus an AGN tem-
plate (Severgninietal.2003). The AGN template, in partic-
Fig.2. 2XMMpJ145442.2+182937: XMM-Newton MOS+pn
ular, has a component (the continuum and the broad emis-
folded spectrum. The best-fit model is a single-absorbed
sion lines) that can be absorbed according to an appropriate
power-lawwithaGaussianline.
extinction curve and a second component (the narrow emis-
sion lines) that is not affected by the absorption.We estimate
the optical absorption as the value that best emulates the ob-
servedspectrum(boththecontinuumandtheemission-linein-
tensity), paying particular attention to correctly reproducing
the strength of some critical lines, like the [OIII]λ5007Åand
theHα.Wefindthattwosources(2XMMpJ145442.2+182937
and 2XMMpJ115121.8−283605) are affected by a relatively
high absorption (A > 2 mag and A ∼ 2 mag), while
V V
thethird(2XMMpJ151612.2+070341)isonlymoderatelyab-
sorbed (A ∼1 mag). When translated into a value of N us-
V H
ing the standardGalactic relation(e.g.Maiolinoetal.2001a),
the absorption levels inferred from the optical spectra are
NH∼> (2−4)×1021cm−2,i.e.lower(significantlylower,inthe
case of 2XMMpJ151612.2+070341)than the values obtained
fromtheX-raydata(see§3).
Theexistenceofa fractionofAGNwherethelevelofab-
sorption deduced by the X-ray spectra is significantly higher
Fig.3. 2XMMpJ151612.2+070341: XMM-Newton MOS+pn
thanwhatcanbeinferredfromtheopticaldataiswellknown
foldedspectrum.Thebest-fitmodelisaleaky-absorbedpower-
and discussed in the literature (e.g. Maccacaroetal.1982;
law.
Maiolinoetal.2001b; Cappietal.2006). To date, we are not
inapositionofquantifyingthefrequencyofthistypeofsource
since thesourcesdiscussedheredonotconstitutea complete,
Long-slit spectroscopy was carried out in the 3800−9000
well-defined sample of objects. It is very likely that the way
Å band with a low dispersion (∼ 1.7 Å/pixel), long-slit con-
wehavechosentheseobjectsoutofthe2XMMpcatalogue,in
figurationandwithalow-resolutiongrism(Grism#2,R∼600).
particular the requirementof a bright optical counterpart,has
Theseeingduringtheobservingrunrangedfrom1”to2”,and
favouredtheselectionofobjectsthatarehighlyabsorbedinthe
weusedaslitwidthof1”–1.5”.Datareductionwasperformed
X-ray,butwithalow/moderateopticalabsorption.Alargerand
usingIRAF4 standardpackages.Wavelengthcalibrationswere
unbiasedsample ofHR-selectedsourceswouldbeinstrumen-
carriedoutbycomparisonwithexposuresofHe-Arlamps.The
talinevaluatingthefractionofsourcesstronglyabsorbedinthe
relativefluxcalibrationofthespectrawasobtainedusingobser-
X-raysbutonlymoderatelyabsorbedintheopticalband.
vationsofspectro-photometricstandardstarsperformedwithin
afewhoursofthespectroscopyofoursources;noattemptwas
madetoobtainanabsolutefluxcalibrationofthespectra. 5. Concludingremarks
The optical spectra of the three objects are presented in
Threesourcesofthe2XMMpcataloguewithhardX-raycolour
Figs. 4, 5, and 6, (right panels). The spectra show relatively
strongly suggesting the presence of absorption were also ob-
4 IRAF is distributed by the National Optical Astronomy [email protected]
Observatory,whichisoperatedbytheAssociationofUniversitiesfor (NH > 1022 cm−2)intheX-raybandandmoderateabsorption
Research in Astronomy, Inc., under cooperative agreement with the (AV ∼ 1−2 mag) in theoptical. We canthereforedefinitely
NationalScienceFoundation. state that we are dealing with a successful selection criterion
E.Memolaetal.:SearchingforabsorbedAGNinthe2XMM-Newtonpre-releaseEPICSerendipitousSourceCatalogue∗ 7
ableto“isolate”obscuredsourcesinX-rays.Pageetal.(2007)
havebeenstudyinghard-spectrumXMM-Newtonsourceswith
2−4.5keV X-rayflux > 10−14 erg cm−2 s−1 and get the same
highefficiencyinfindingabsorbedX-raysources(6.3×1021 <
N <2.5×1023cm−2).
H
We note that, of the large number (427) of high-latitude
sources with HR3 > 0 and ML > 20 available in the 2XMM
pre-releaseSerendipitousEPICSourceCatalogue,381havea
MOS2X-rayfluxinthe4.5−12keVbandlargerthan10−13erg
cm−2s−1.Thismeansthattheiropticalspectroscopyshouldbe
achievablewithground-basedtelescopes,asalreadyhappened
formostofthesourcesoftheHBS.Wecanthereforeforeseea
valuableincrementtotheknownsampleofabsorbedAGN,up
to afewhundrednewlyabsorbedAGN,andabout1/3among
them, as inferred from the HBS statistics, should be the rare
type2quasars.
Acknowledgements. Weacknowledgefinancialcontributionfromthe
contractASI-INAFI/023/05/0. Partofthisworkwassupportedbya
Cofingrantfortheyear2006.
References
Arnaud,K.A.1996,ASPC,101,17
Avni,Y.1976,210,642
Brandt,W.N.,&Hasinger,G.2005,ARAA,43,827
Caccianiga,A.,Severgnini,P.,Braito,V.,etal.2004,A&A,416,901
Cappi,M.,Panessa,F.,Bassani,L.,etal.2006,A&A,446,459
Cash,W.1979,ApJ,228,939
DellaCeca,R.,Maccacaro,T.,Rosati,P.,etal.2000,A&A,355,121
DellaCeca,R.,Ballo,L.,Tavecchio,F.,etal.,2002,ApJ,581,L9
DellaCeca,R.,Maccacaro,T.,Caccianiga,A.,etal.2004,A&A,428,
383
Della Ceca, R., Caccianiga, A., Severgnini, P., et al. 2005, Proc. of
The X-ray Universe 2005, San Lorenzo de El Escorial (Spain)
organizedbyESA-ESAC,astro-ph/0510845
Fabian,A.C.,&IwasawaK.1999,MNRAS,303,L34
Maccacaro,T.,Perola,G.C.,Elvis,M.1982,ApJ,257,47
Maiolino,R.,Marconi,A.,Salvati,M.,etal.2001a,A&A,365,28
Maiolino,R.,Marconi,A.,Oliva,E.2001b,A&A,365,37
MonetD.BirdA.,Canzian,B.,Dahn,C.,etal.1998,VizieROnline
DataCatalog,1252
Page,M.J.,Lehmann,I.,Boller,Th.,etal.2007,submittedtoMNRAS
Severgnini,P.,Caccianiga,A.,Braito,V.,etal.2003,A&A,406,483
Turner,T.J.,George,I.M.,Nandra,K.,etal.1997,ApJ,488,164